TRAIL/S-layer/graphene quantum dot nanohybrid enhanced stability and anticancer activity of TRAIL on colon cancer cells

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), known as a cytokine of the TNF superfamily, is considered a promising antitumor agent due to its ability to selectively induce apoptosis in a wide variety of cancer cells. However, failure of its successful translation into clinic has led to development of nano-based platforms aiming to improve TRAIL therapeutic efficacy. In this regard, we fabricated a novel TRAIL-S-layer fusion protein (S-TRAIL) conjugated with graphene quantum dots (GQDs) to benefit both the self-assembly of S-layer proteins, which leads to elevated TRAIL functional stability, and unique optical properties of GQDs. Noncovalent conjugation of biocompatible GQDs and soluble fusion protein was verified via UV–visible and fluorescence spectroscopy, size and ζ-potential measurements and transmission electron microscopy. The potential anticancer efficacy of the nanohybrid system on intrinsically resistant cells to TRAIL (HT-29 human colon carcinoma cells) was investigated by MTT assay and flow cytometry, which indicated about 80% apoptosis in cancer cells. These results highlight the potential of TRAIL as a therapeutic protein that can be extensively improved by taking advantage of nanotechnology and introduce S-TRAIL/GQD complex as a promising nanohybrid system in cancer treatment.

One approach that has been extensively utilized in prior studies is fusing TRAIL with different tags or other proteins to facilitate its purification and trimerization and also to enhance its stability and bioactivity, which consequently leads to improvement of its therapeutic efficacy. Wiley et al. in 1995 designed the first recombinant version of TRAIL with an added FLAG epitope tag 24 . This approach was followed by adding other tags, motifs and domains to the extracellular domain of TRAIL [25][26][27] , fabricating various fusion proteins [28][29][30][31] , or by trying to set up innovative approaches to modify TRAIL purification [32][33][34][35] . However, despite all valuable progress in improving bioactivity, TRAIL purification still suffers from aggregation and formation of inclusion bodies, requiring successive steps of denaturation and renaturation of proteins and complicated multistep purification procedures. The novel S-TRAIL fusion protein could successfully overcome all the challenges facing the expression and purification of TRAIL, via a simplified expression system and one-step Ni-affinity chromatography with a yield of up to 8 mg/L of bacterial culture, which is comparable with other studies in this field 35,36 . GQD characterization. UV-vis spectroscopy of synthesized GQDs revealed two characteristic absorption bands at 234 and 337 nm (Fig. 3a) related to the π → π * and n → π * transitions of C=C and C=O 37,38 . The prepared GQDs exhibited a strong blue emission at 440 nm ( max ) upon excitation at 300-420 nm; the maximum emission intensity was obtained by excitation at 360 nm (Fig. 3b) This photoluminescence band is excitation-independent, meaning that max does not shift with the excitation wavelength, which could imply that both the size and the surface hybridization state ( sp 2 clusters) of GQDs are uniform 39 . The FT-IR spectrum of GQDs revealed characteristic stretching bands of O-H and N-H (3000-3500 cm −1 ), vibrational bands of C=O   www.nature.com/scientificreports/ in COOH and CONH (1709, 1667 cm −1 ), and bending vibrations of C=C and C-N (1575-1401 cm −1 ), as shown in Fig. 3c.

Characterization of S-TRAIL/GQD. For S-TRAIL/GQD formation, optimization in terms of concen-
trations of S-TRAIL and GQDs, incubation time and temperature was done (data not shown). Based on the obtained results, efficient complex formation was accomplished by mixing S-TRAIL (final concentration of 160 µg/ml, pH 7.4) and GQDs (final concentration of 6 µg/ml, pH 7) at the ratio of 1:1 V/V at 25 °C for 30 min under constant gentle shaking in the dark. S-TRAIL/GQDs were then separated by centrifugation from nonconjugated S-TRAILs and GQDs (Fig. 4a). As shown in Fig. 4b, c, the presence of S-TRAIL and GQDs in the pellet was confirmed by SDS-PAGE and UV-Vis spectroscopy. The absorption peaks at 280 nm and 337 nm are related to the S-TRAIL and GQDs, respectively (Fig. 4c). In addition, the fluorescence emission of the pellet (S-TRAIL/GQD) confirmed the presence of GQDs in the precipitated complexes (Fig. 4d). The conjugation of GQDs with S-TRAIL was further investigated by recording the emission spectra of GQDs in the presence of different concentrations of S-TRAIL (Fig. 4e). Concentration-dependent quenching of GQD fluorescence is the result of GQD interactions with S-TRAIL. By conjugation of GQDs to S-TRAIL, a decrease in ζ-potential from its initial value of -24.9 ± 2.9 mV observed, which could be attributed to electrostatic interactions of hydroxyl and carboxylic acid functional groups of GQDs with positively charged residues of S-layer part of S-TRAIL (Fig. 4f). Changes in intrinsic fluorescence of S-TRAIL in the presence of different concentrations of GQD are also indicative of the presence of interactions between GQDs and S-TRAIL (Fig. 4g).
Furthermore, TEM images of the S-TRAIL, GQD and the S-TRAIL/GQD (Fig. 5) showed that GQDs were in a narrow size distribution with mean diameters of 2.01 ± 0.28 nm (Fig. 5b, d). While the S-TRAIL appeared as amorphous aggregates (Fig. 5a), the S-TRAIL/GQDs were seen as spherical nanoparticles with a mean diameter of 17.77 ± 2.03 nm (Fig. 5c, e). The use of S-TRAIL/GQDs for cancer treatment would take advantage of their preferential accumulation in tumors owing to the enhanced permeability and retention in the tumor site as well as the possibility of tracking and imaging of the delivered system.

In vitro cytotoxicity and bioactivity assessments.
In spite of the fact that TRAIL has known as a potent anticancer agent, many cancer cells such as non-small cell lung cancer (NSCLC) 40,41 , prostate cancer 42 , triple-negative breast cancer (TNBC) 43 and colorectal cancer (CRC) 44,45 have shown resistance to TRAIL. To address this issue, one proposed strategy was combination therapy (pre-treatment or co-treatment) with chemotherapeutic drugs like cisplatin, doxorubicin (DXR) and 5-fluorouracil to sensitize tumor cells to TRAIL 44,46 . Accordingly, in the present study, HT-29 cells were pre-treated with DXR to sensitize tumor cells and take advantage of TRAIL and DXR synergistic effects mostly through enhancement of pro-caspase-8 activation and mitochondria-dependent death pathway (Bid cleavage) 44,47 MTT assays were used to evaluate the cytotoxic effects of GQDs, S-layer, S-TRAIL and S-TRAIL/GQD with doxorubicin pretreatment ( DXR + ) and without doxorubicin pretreatment ( DXR − ) on the HT-29 human colon carcinoma cell line. GQDs, DXR and S-layer proteins at concentrations that were used for fabrication of the complex had negligible cytotoxic effects on target cells ( Fig. S1a-c). It should be mentioned that the S-layer protein concentrations in this experiment were equal to the molar concentration of the S-layer part in the S-TRAIL cytotoxicity assays (Fig. 6a).Concentration-dependent cytotoxic effects of the S-TRAIL and S-TRAIL/GQD on the cells could be clearly seen (Figs. S1d and 6a). The highest tested concentrations of S-TRAIL and S-TRAIL/GQD decreased the viability of DXR-pretreated cells to 20.32 ± 0.02 and 18.99 ± 0.02%, respectively. One plausible explanation of the higher bioactivity of the S-TRAIL/ GQD compared to the S-TRAIL, particularly at lower concentrations ( Fig. 6a, b), is that GQDs could provide a surface for self-assembly of the S-TRAIL, facilitating the oligomerization of TRIAL monomers. The cytotoxic activity of S-TRAIL and the S-TRAIL/GQD at higher concentrations was not significant, which could be due to the saturation of the cell surface receptors for TRAIL. The results also demonstrated that DXR pretreatment caused a significant increase in cytotoxicity of the S-TRAIL and the S-TRAIL/GQD, which confirmed the undeniable role of DXR pretreatment to overcome therapeutic resistance to TRAIL. HFF-1 cells which have been reported as TRAIL resistant cells 48 could retain their viability upon S-TRAIL treatment at different concentrations (Fig. S2) and no significant difference in viability of DXR + and DXR − cells was seen.
The apoptotic activity of S-TRAIL with/without DXR pretreatment in HT-29 cells were verified by flow cytometry (Annexin V-FITC/PI staining assay) 49 and compared to untreated cells (Fig. 7a). The cells only treated with doxorubicin showed 16.77% apoptosis (Fig. 7b). Figure 7c clearly demonstrated the apoptotic activity of S-TRAIL. As expected, pretreatment of the cells by DXR significantly increased the population of apoptotic cells from 29.8 to 83.2% of total cells (Fig. 7d, e). The results revealed the potential of sequential therapy with DXR and S-TRAIL to efficiently induce apoptosis in TRAIL-resistant cancer cells.
S-TRAIL stability study. As mentioned earlier, one of the main limitations of TRAIL as a therapeutic agent is its low solubility and stability. Here, the stability of the purified S-TRAIL and TRAIL after 1, 2 and 4 weeks of storage at 4 °C was compared by analyzing their cytotoxic effects on HT-29 cells (Fig. 8). Interestingly, while TRAIL activity was notably diminished after two weeks of storage at 4 °C, the S-TRAIL almost entirely retained its cytotoxic activity even after 4 weeks.
The increased stability and bioactivity of TRAIL by its conjugation to other elements, such as self-assembled albumin nanoparticles 50,51 , hyaluronic acid 5,52 , elastin-like polypeptides (ELPs) 30 or diphenylalanine (FF) 53 , has been reported earlier. These strategies generally require sophisticated conjugation methods and purification procedures. In this study, the cytotoxic S-TRAIL that was efficiently expressed and purified by a simple method showed enhanced bioactivity and storage stability. were observed by using fluorescence microscopy and compared with control and GQDs treated cells (Fig. 9a-c). TRAIL tracking has been reported earlier by creating fusion proteins of TRAIL and green fluorescent protein 54  www.nature.com/scientificreports/ and firefly luciferase 55 . In another study, TRAIL was conjugated with magnetic nanoparticles, which enabled MRI imaging of the treated cells 56 . To the best of our knowledge, this is the first report of tracking TRAIL by taking advantage of GQD fluorescence.

Conclusion
In this study, a multifunctional nanohybrid system was fabricated by taking advantage of the apoptotic properties of TRAIL, self-assembly of the S-layer proteins and unique optical properties of GQDs. S-TRAIL that was produced by a simple procedure, showed enhanced solubility, stability and bioactivity in comparison with TRAIL. The conjugation of the S-TRAIL with GQDs further enhanced its apoptosis-inducing activity in human colon carcinoma cells. These findings strongly suggest that the S-TRAIL/GQD could be considered as a potent and promising candidate for cancer treatment.

Preparation and characterization of TRAIL, S-layer and S-TRAIL. Construction of S-TRAIL expres-
sion vectors. Expression vectors were constructed by inserting the product of SOEing PCR which connected PCR fragments of TRAIL and SlpA, into the vector pET-28a(+).cDNA of extracellular domain of the human TRAIL (residues 114-281) was made from total RNA extracted from human peripheral white blood monocytes. The template for SlpA (residues 1-445) amplification was the genomic DNA extracted from L. acidophilus. TRAIL and SlpA genes were linked using a flexible linker (GGGGSGG). In this regard, four primers, (Table 1) were designed to amplify TRAIL-linker-SlpA. TF and TR were used to amplify the TRAIL-linker and SF and SR were used to amplify the linker-SlpA genes. The PCR products containing TRAIL-linker and linker-SlpA were mixed together for another 15 cycles of PCR without adding any primers. Following construction of TRAILlinker-SlpA fragment, amplification was carried out using TF and SR primers. TRAIL-linker-SlpA was introduced into the pET-28a(+) at the NheI and SalI restriction sites and E. coli BL21(DE3) was transformed with the vectors for the protein expression. Synthesis and characterization of GQDs. N-doped GQDs were prepared using the hydrothermal method by pyrolysis of citric acid and urea according to a reported procedure 37 . Briefly, 0.21 g citric acid and 0.18 g urea were dissolved in 5 ml Milli-Q water. Then, the solution was transferred into a 100 ml Teflon lined stainless steel autoclave and heated at 160 °C in an electric oven for 3 h. After cooling to room temperature, 10 ml ethanol was added to the solution, and centrifugation was performed at 4000 × g for 10 min. The supernatant was dried by a rotary evaporator, and the dried GQDs were dispersed in water and stored in the dark at room temperature until further characterization and use. Absorption and FTIR spectra of the synthesized GQDs were measured by UV-visible spectroscopy (Perki-nElmer, Lambda 25) and FTIR spectroscopy (Tensor 27 FT-IR spectrometer), respectively. Fluorescence spectra were recorded by a Cytation TM 3 multimode detection system (BioTek) at room temperature. The ζ-potential of the GQDs was determined by a dynamic light scattering Zetasizer Nano ZS instrument (Malvern Instruments, UK). TEM images of GQDs were taken on carbon-coated grid Cu Mesh 300 (TEM, Zeiss-EM10C-80 kV).

Conjugation of S-TRAIL and GQDs. Conjugation of the S-TRAIL and GQDs was accomplished based
on adsorption of the oppositely charged S-TRAIL on negatively charged GQDs. To determine the optimum condition of conjugation, different concentrations of S-TRAIL (40, 80, 160, 320 and 640 µg/ml) were mixed with different concentrations of GQDs (3, 6, 12 and 24 µg/ml) at different temperatures (4 °C, 25 °C, 37 °C) and incubation times (15 min to 3 h). To remove non-conjugated S-TRAILs and GQDs, centrifugation at 36,000 g for 30 min at 25 °C (Sigma 3-30 KS centrifuge) was carried out, and the pellet was collected and re dispersed in appropriate buffer.
Conjugation of the S-TRAIL and GQDs was confirmed by polyacrylamide gel electrophoresis (12.5%), UV-visible spectroscopy, intrinsic fluorescence spectroscopy of GQDs and S-TRAIL using a Perkin-Elmer LS-55 fluorescence spectrometer (Perkin-Elmer, USA), ζ-potential measurements and TEM imaging.
Cells were seeded onto 96-well polystyrene culture plates at a density of 1 × 10 4 cells per well and incubated for 24 h. The cytotoxicity of S-TRAIL and S-TRAIL/GQD at different concentration with doxorubicin pre-treatment ( DXR + ) and without doxorubicin pretreatment ( DXR − ) were evaluated using MTT assay. For doxorubicin pretreatment, the cells were pretreated with sub-toxic concentrations of doxorubicin (0.1 µM) for 24 h. Then, the culture media were replaced by fresh RPMI and were subsequently treated with different concentrations of S-TRAIL and S-TRAIL/GQD for 24 h. Then the viability of the cells was evaluated using MTT 57 . Flow cytometric analysis of the treated cells. Apoptotic effects of S-TRAIL on the cells with doxorubicin pre-treatment ( DXR + ) and without doxorubicin pretreatment ( DXR − ) were assessed by Annexin V-FITC/ PI flowcytometry 49 . HT-29 cells were cultured in 12-well tissue culture plates ( 8 × 10 4 /well ) and allowed to adhere overnight. For doxorubicin pretreatment, the cells were pretreated with sub-toxic concentrations of doxorubicin (0.1 µM) for 24 h. Then, the culture media were replaced by fresh RPMI and were subsequently treated with S-TRAIL for 12 h. The untreated cells and DXR only treated cells were considered as controls. The cells were then stained with Annexin V-FITC/PI and analyzed using a BD FACSCalibur flow cytometer (BD Biosciences, San Jose, CA, USA), with 10,000 events counted per sample. The percentage of apoptotic cells was calculated by FlowJo software.
S-TRAIL stability study. The cytotoxic activity of TRAIL and S-TRAIL on HT-29 cells was compared at different time intervals after their production using MTT assay. In this regard, HT-29 cells which were pretreated Table 1. Sequence of primers required for construction of trail-linker-SlpA fragment, cloned in pET-28a (+) plasmid; highlighted part indicates linker sequence. TF  TAA CAG CTA GCA TGG TGA GAG AAA GAG GTC CTC AGA GAG TAG CAG   TR  TCC ACC TGA TCC TCC ACC TCC ACC AAC TAA AAA GGC CCC GAA AAAAC   SF  GGA GGT GGA GGA TCA GGT GGA ATG AAG AAA AAT TTA AGA ATC GTT AGC   SR  AAT ATG TCG ACT TAT CTA AAG TTT GCA ACC TTA ACG TAAG